The proposed research is directed toward acquiring a better understanding of the transfer of solutes and water across the renal tubules with emphasis on the control mechanisms exerted by factors acting on the peritubular side of the epithelium. The nature of the effect of capillary absorption on epithelial transport and its role in glomerulo-tubular balance will be examined. Electrophysiological techniques shall be employed to evaluate the role of ionic diffusion in the removal of the reabsorbate. The permeability of peritubular capillaries to protein will be studied. Micropuncture methods will be employed to test whether changes in bicarbonate concentration in peritubular fluid play a role in the natriuresis which occurs during acute hyperkalemia.